Measurements of airflow and volume are regularly performed during clinical investigations in respiratory physiology and pathology, occupational medicine, sports medicine, allergy and immunology, respiratory function laboratories, pulmonology, as well as in a variety of industrial and scientific settings. Spirometry is a non-invasive method of lung function testing that measures the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry generates measurements that are helpful in assessing conditions such as asthma, chronic bronchitis, emphysema, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, cystic fibrosis, and other such lung related conditions.
For example, the incidence of chronic obstructive pulmonary disease (COPD) has escalated in recent years and is now the fourth leading cause of death in the US, at 121,000 fatalities per year. Prevalence of severe COPD among individuals aged 40 and older is estimated to be at 10%. The prevalence of asthma is even higher. The cost to the health care system of dealing with COPD was estimated at over 32 Billion US Dollars in 2002. (European Union Conference on Chronic Respiratory Disease: Purpose and Conclusions, M. Decramer et al.) Effectively managing COPD requires early detection and prediction, affecting the natural course of the disease, and improving management with optimized intervention.
Spirometry measurements are usually performed in two distinct ways: via a volume measurement device, or via a flow measurement device. In operation, spirometry measures the volume and/or flow of air that is exhaled or inhaled. The spirometry test is performed using a device called a spirometer that displays graphs called spirograms. Spirograms include a volume-time curve showing the volume, e.g. liters, on the y-axis and time, e.g. seconds, along the x-axis. The spirograms also include a flow-volume loop that graphically depicts the rate of airflow on the y-axis and the total volume inhaled and exhaled on the x-axis.
Previously, volume measurements were performed in a number of ways, using a “bell” spirometer or a “bag-in-box” device where an inflatable element, i.e. “bag”, is enclosed in an air-tight element, i.e. “box” that is physically connected to external air through a flow meter. The purpose of the box was not only to contain the bag, but also to serve as a primary measurement device, where the volume contained by the bag is determined as a function of the pressure or residual volume in the box. The bag-in-box is no longer used because the device is bulky, leaks, and requires airproofing after the internal bag is changed.
Referring to FIG. 1A there is shown a prior art “bell” volume spirometer, in which a patient is starting to exhale into mouthpiece, the flexible tube, the inner tubing of the spirometer and the movable bell. The volume spirometer includes a moving chamber 12 or “bell”, a breathing tube 14, and a mouthpiece 16.
Referring to FIG. 1B there is shown the prior art volume spirometer of FIG. 1A after the patient has completed exhaling and a single-breath measurement with no circulation through a soda lime receptacle. In FIG. 1B the volume spirometer includes a valve 18 and the similar components of the volume spirometer of FIG. 1A.
In FIG. 1C, there is shown the air flow during rebreathing and the chemical process for CO2 removal. In the rebreathing system, the CO2 exhaled by the patient does not increase in the bell because the CO2 is removed by the soda lime and replaced with O2.
Referring now to FIG. 1D, there is shown a prior art rebreathing system that uses the movable bell of FIG. 1A. The rebreathing system includes the movable bell, an inlet valve represented as a red valve, an outlet valve represented as a blue valve, a soda lime receptacle, an electric pump, and variety of different tubes.
The volume-measurement spirometer measures the volume of a particular gas entering the moving bell that rises and falls under the competing action of gravity and pressure from the gas pushed inside the chamber during the measurement session. The change in volume is typically measured electromechanically by encoders electrically coupled to transducers and suitable electronic circuitry or digital hardware and software that convert the measurements of displacement of the bell into measurements of volume within the chamber. Although this instrumentation is mature, reliable, and precise, the spirometric instrumentation is cumbersome and not very portable. Additionally, even if the bell spirometer is movable, it must be calibrated and retested after being moved.
There are also additional components and actions that must be taken to sanitize the volume-measurement spirometer. Since different patients breathe in and out of the same instrument, this results in the potential spread of disease and precautionary measures must be taken including using filters and membranes, and periodically sterilizing the spirometer. These sanitary measures also add to the expense of the operation and maintenance of the traditional spirometer.
The flow-measurement spirometer measures flow, not volume, and obtains a volume measurement by post-processing flow data. The flow-measurement spirometer includes peak-flow meters and pneumotachographs. The flow-measurement spirometer devices are smaller and more portable. However, the flow-measurement spirometer is also more sensitive to drift and accumulation errors, and is considered less accurate, precise and reliable than the volume-measurement spirometer.
In operation, the volume-measurement and flow-measurement devices are typically placed in contact with the patient by providing a disposable mouthpiece that is usually made of cardboard, but often also plastic. The patient proceeds to hold either a flexible tube or mask for volume device, or the flow device itself and then perform the test procedure that may involve re-breathing. The disposable mouthpiece is then replaced after patient use.
When the spirometer is used, the patient exhales an air mixture with droplets of saliva and pathogens that come into contact with various working components of the spirometer that are not disposed of. For example, the flexible tube and moving chamber that comes in contact with the patient's exhaled breath is not disposed of after patient usage.
The next patient that uses the spirometer may inhale pathogens or droplets of saliva from either the chamber or through the flow sensor. This phenomenon is minimized, but not eliminated, by the use of filters as well as periodic cleaning and sterilization of the entire instrument ensemble. Additionally, the cost of periodically sterilizing the spirometer is significant, so the sterilization operation cannot be performed after every patient. Furthermore, even if cleaning was performed after each patient, there is no guarantee that spirometer instrumentation would be sterile. Further still, the measurement devices must also be cleaned and disinfected periodically in order to avoid growth of molds and pathogens; and this cleaning procedure requires the measurement device to be disassembled and re-calibrated.
In summary, a sterile environment cannot be ensured by simply replacing the cardboard mouthpiece of either the volume measurement device or flow measurement device.